Having examined the dental characters of existing species of the Horse-kind so far as seemed requisite for the determination of the Equine remains found in the Cavern of Bruniquel, I have been enabled, on the same basis of comparison, to deal with other Equine fossils, and propose to communicate in the present Paper the results of this labour in elucidation of those which have reached me from some American localities. In the account of the Mammalian Remains brought from South America in the ‘Voyage of the Beagle,’ I described and figured an upper molar tooth as belonging to a species of Equus ; and this tooth, having been found by Mr. Darwin imbedded in the quartz shingle of cemented pebbles at Punta Alta in Bahia Blanca, together with remains of Megatherium, Megalonyx, Mylodon , and Scelidotherium , I concluded not to be a tooth of a horse imported by Europeans into South America, but to have belonged to an Equine species which had coexisted with those large Megatherioids and had, with them, become extinct at a prehistoric period. An Equine upper molar tooth of similar pattern, included in the collection of fossils from the same ‘Voyage,’ was labelled as having been found in red argillaceous (postpliocene ?) deposits at Santa Fé, in the province of Entre Rios, Buenos Ayres; and this tooth being associated in the series with parts of Mastodon and Toxodon confirmed me in the above conclusion.